Frequency metrology in ultracold helium

For decades, precision spectroscopy of simple atomic systems has been used to test and refine our understanding of quantum electrodynamics (QED). In the latest contribution to that effort, researchers led by Wim Vassen (Vrije University Amsterdam) have measured—to a precision of 8 parts in 10¹²—an exceedingly weak IR transition between the triplet and singlet metastable states of atomic helium, 2 ¹ S ₀ and 2 ³ S ₁. So highly forbidden is the excitation, which violates spin and parity selection rules, that it has never been seen, much less measured to high precision. Earlier spectroscopy of He exploited atomic beams or gas discharge cells, but the Amsterdam experiment combines optical trapping and frequency comb technology. The trap cools a few million He atoms to a near standstill, effectively eliminating Doppler broadening, and allows them to interact with excitation light for up to six seconds; the comb, in turn, provides a rock-solid laser-frequency calibration. Any excited atoms are expelled from the trap, and when the trap is turned off the ones remaining fall onto a microchannel plate that counts them. A plot of that number as a function of applied laser frequency reveals the transition frequency with an experimental error three orders of magnitude lower than is now possible from state-of-the-art QED calculations. The Amsterdam group also used the frequency difference measured between ³He and ⁴He isotopes to determine the nuclear charge radius for ³He (1.961 ± 0.004 fm), which provides a stringent test for nuclear theory. (R. van Rooij et al., Science 333, 196, 2011 .)—R. Mark Wilson